The present invention relates to techniques for changing the speed of an electric motor, such as bringing the motor to a gradual controlled stop; and particularly to such techniques and the apparatus for practicing them which employ a combination of dynamic braking and "cycle-skipping" speed control.
When an electric motor drives components of automated manufacturing equipment, the motor often must be precisely controlled to accurately position a workpiece with respect to other components of the manufacturing equipment. For example, an assembly line may transfer a workpiece from one work station to another using a conveyor belt or similar apparatus. As the workpiece nears the next station, the motor must be stopped so that the assembly line positions the workpiece accurately with respect to the next work station.
In order to achieve maximum operating efficiency of the assembly line, it is desirable to transfer the workpieces as fast as possible between the different work stations. However, the higher the motor speed, the greater the inaccuracy in stopping the motor to position the workpiece at the next station. Therefore, the designer of an assembly line control system is left with a trade-off between a high workpiece transfer rate with a relatively low positioning accuracy and increasing the positioning accuracy by slowing the workpiece transfer rate.
Various types of motor braking techniques have been employed to rapidly stop an electric motor in a manner which provides some degree of control over that operation. An example of such a device is disclosed in U.S. patent application Ser. No. 07/103,729 entitled "Apparatus and Method for Braking an Electric Motor" and assigned to the same assignee as the present invention. This type of dynamic braking applied pulses of the alternating current from a motor supply through the motor windings at selected points in time to produce an electromagnetic force that opposed the electromagnetic force due to the magnetism of the motor's rotor. The opposing magnetic fields generated a negative torque within the motor which slowed its speed.
Although such forms of dynamic braking provided a greater degree of control over the positioning of workpieces than was achieved by merely allowing the motor to coast to a stop, a certain degree of inaccuracy still existed when the workpieces were being transferred at a relatively high speed. Depending upon the tolerances required for the processing along the assembly line, even when dynamic braking was employed, an unacceptably large positioning tolerance could exist. In addition, dynamic braking alone may not provide as smooth a slowing of the motor as is required.
Assembly lines typically use AC induction motors in which the speed of operation is synchronized to the frequency (50 or 60 Hz.) of the alternating current supplied to the motor. Merely controlling the voltage or current applied to the motor does not provide an effective way to alter its speed, since the speed is dependent upon the frequency of the alternating current which remains constant despite fluctuations in the voltage until the motor stalls. In order to control the speed of an induction motor, various techniques for changing the frequency of the current applied to the motor have been devised. Many of the these techniques involve relatively complex electronic control circuits for converting the standard alternating current supply frequency into different frequencies for controlling the speed of the motor.
A technique commonly referred to as "Cycle-Skipping" was developed, as an alternative to the relatively elaborate and expensive A.C. frequency conversion apparatus. In this technique, thyristors couple the source of alternating current to the motor and are switched at proper points in time to generate a fundamental frequency component of the alternating supply current. An example of this cycle-skipping method is disclosed in U.S. Pat. No. 4,176,306 entitled "Speed Control Apparatus." The technique described in this patent triggers the thyristor for a phase line of the alternating current supply during several consecutive positive half-cycles of the A.C. voltage for that supply line, and then it is not triggered for one or more cycles of the supply voltage. Next, the thyristor is triggered during several consecutive negative half-cycles of the supply line voltage. This pattern repeats with a pause of one or more cycles between each pattern. The thyristors for the other two phase lines in a three-phase circuit are fired in the same pattern, but 120 degrees out of phase. The pattern applies current to the motor having an effective frequency which is a fraction of the A.C. supply frequency. The motor synchronizes to this lower frequency and runs at a slower speed.
However, merely changing the thyristor firing from occurring every cycle to a cycle-skipping pattern by itself was insufficient to produce a reduction in the speed of the induction motor, since the waveform of the current produced by the cycle-skipping still has a component of the original supply frequency (50 or 60 Hz.). Therefore, in order to break the motor out of synchronism with the A.C. supply frequency, the motor control circuit provided a switch mechanism, such as a contactor, to reverse the connections of the three-phase A.C. supply lines to the motor. The contactor mechanism had to be switched to alter the supply line connections to the motor according to the mode at which the thyristors were being controlled.